Various methods are known for descaling hot metal strip (or sheet) in a continuous annealing line of a production facility. One method for descaling metal strip is generally referred to as "fused alkali treatment", in which the undesired oxide coatings on the exterior surfaces, which are formed during the rolling or annealing procedures, are removed using fused (or molten) alkali salts for the purpose of improving the strip surface either for further processing, or as an end product. To complete the descaling, the metal strip is further processed, such as for example, by rinsing, quenching, and/or immersion for a short period of time in an acid bath (i.e., "acid pickling"). The fused alkali treatment can be used to descale a variety of alloys of metal strip, such as stainless steel, nickel, cobalt and titanium alloys.
Various types of apparatus have been proposed in the industry for effecting this descaling process. For example, an immersion tank is used wherein metal strip is immersed in a bath of mixed alkali metal hydroxides or salts. To guide the strip into and out of the molten bath, metal rolls typically are used to support the metal strip. Using metal rolls, however, can scratch and mar the surface of the strip through the presence of insoluble particles on the strip and/or the relative movement between the hot strip and the rolls.
Additionally, an immersion tank requires a considerable amount of space in the processing line of the production facility. Further, it has been determined that the descaling salts interact rather quickly with the scale on the strip, in fact typically quicker than the time it takes the steel to travel through the bath. Hence, using an immersion tank to descale a metal strip in a continuous annealing line can be inefficient by "overconditioning" the strip with excessive salt contact. Overconditioning the surface of the strip can make it more difficult to clean the strip in the subsequent acid pickling.
Methods and apparatus have been developed where descaling salts are applied directly to the metal strip in a spray after the strip leaves the annealing furnace. Spray systems generally reduce overconditioning of the strip and can have a scale loosening, scouring or scrubbing action which facilitates oxidizing the scale. For example, one technique is shown in Faler, U.S. Pat. Nos. 3,126,301 and 3,174,491, both of which are owned by the assignee of the present invention, wherein the metal strip is supported by tension through the descaling system. The molten salt in Faler is atomized within a spray box using large quantities of a gas, such as super-heated steam. The steam is passed into nozzles through which is fed small quantities of the molten salts. The nozzles for applying the atomized salt are illustrated as being located centrally of the strip and directed toward opposite surfaces thereof. Steam is also provided through steam nozzles into the atmosphere of the oven for a general heating of the atmosphere to prevent any non-atomized salt from solidifying and contacting the strip.
Another method for descaling metal strip is shown in Hirata, et al, U.S. Pat. No. 4,251,956, in which a descaling slurry is applied to a surface of the strip from a nozzle array. The nozzle array comprises four sets of nozzles mounted between a supporting column and a connecting rod. The nozzle array is positioned in spaced relation to the strip surface and is directed toward the strip surface at an acute spray angle relative to the direction of movement of the strip.
Still other spray descaling systems using a plurality of nozzles located across the width of the strip are shown in McClanahan, et al, U.S. Pat. No. 4,361,444, and Hiroshima, U.S. Pat. No. 3,617,039.
Although the above-described descaling systems provide certain benefits in spraying salt across the surfaces of the metal strip for descaling, as well as reducing or eliminating metal rolls contacting the strip, these systems are not without drawbacks. For example, some of these systems require a large array of nozzles which are located at various orientations relative to the surfaces of the strip, such as shown in Hirata. However, providing a large array of nozzles increases the over-all cost of the descaling system, as well as increases the possibility that one or more of the nozzles will become damaged or clogged.
Moreover, none of the previous descaling systems provide nozzles which can be simply and easily removed from the descaling system, inspected, and repaired or replaced if necessary. As indicated above, the nozzles can have a tendency to become damaged or clogged when used over long periods of time, and it is sometimes necessary to access the nozzles (and related equipment such as pumps and heaters), such that repair or replacement can be made. However, when nozzles are connected to pipes mounted over and traversing the metal strip (such as shown in Hirata), access to the nozzles can be limited and difficult.
In any case, there is a constant demand in the industry for descaling systems which effectively and efficiently descale metal strip in a continuous annealing line of a production facility.